Highly efficient compact head-mounted display system having small input aperture

ABSTRACT

An optical device includes a light-transmitting substrate, input and output apertures, eye-motion box, intermediate element outside of the substrate for coupling light waves into the substrate through the input aperture, a first reflecting surface between two major surfaces of the light-transmitting substrate for reflecting the coupled-in light waves to effect total internal reflection from the major surfaces of the substrate, a second flat reflecting surface parallel to the first reflecting surface located between the major surfaces of the light-transmitting substrate, for coupling light waves out of the substrate, and an optical element for redirecting light waves coupled-out from the substrate through the output aperture, into the eye-motion-box. The input aperture is substantially smaller than the output aperture, active areas of the first and second reflecting surfaces are similar, and each of the coupled light waves covers the entire aperture of the eye-motion-box.

FIELD OF THE INVENTION

The present invention relates to substrate-based light wave guidedoptical devices, and particularly to devices which include reflectingsurfaces carried by a light-transmissive substrate and an array ofpartially reflecting surfaces which is attached the substrate. Theinvention can be implemented to advantage in a large number of imagingapplications, such as, head-mounted and head-up displays, as well ascellular phones, compact displays, and 3-D displays.

BACKGROUND OF THE INVENTION

One of the important applications for compact optical elements is inhead-mounted displays (HMDs), wherein an optical module serves both asan imaging lens and a combiner, in which a two-dimensional display isimaged to infinity and reflected into the eye of an observer. Thedisplay can be obtained directly from either a spatial light modulator(SLM), such as a cathode ray tube (CRT), a liquid crystal display (LCD),an organic light emitting diode array (OLED), a scanning source andsimilar devices, indirectly, by means of a relay lens, or an opticalfiber bundle. The display comprises an array of elements (pixels) imagedto infinity by a collimating lens and transmitted into the eye of theobserver by means of a reflecting or partially reflecting surface actingas a combiner for non-see-through and see-through applications,respectively. Typically, a conventional, free-space optical module isused for these purposes. As the desired field-of-view (FOV) of thesystem increases, such a conventional optical module becomes larger,heavier and bulkier, and therefore, even for a moderate performancedevice, is impractical. This is a major drawback for all kinds ofdisplays but especially in HMDs, wherein the system should be as lightand compact as possible.

The need for compactness has led to several different complex opticalsolutions, all of which, on the one hand, are still not sufficientlycompact for most practical applications, and on the other hand, suffermajor drawbacks in terms of manufacturability, price and performance.

The teachings included in Publication Numbers WO02017/141239,WO2017/141240, WO2017/141242, and PCT/IL2018/051105 are hereinincorporated by reference.

SUMMARY OF THE INVENTION

The present invention facilitates the provision of compact substratesfor, amongst other applications, HMDs. The invention allows relativelywide FOVs together with relatively large eye-motion box (EMB) values.The resulting optical system offers a large, high-quality image, whichalso accommodates large movements of the eye. The optical systemaccording to the present invention is particularly advantageous becauseit is substantially more compact than state-of-the-art implementations,and yet it can be readily incorporated, even into optical systems havingspecialized configurations.

A broad object of the present invention is, therefore, to alleviate thedrawbacks of state-of-the-art compact optical display devices and toprovide other optical components and systems having improvedperformance, according to specific requirements.

In accordance with the present invention there is therefore provided anoptical device comprising an optical device, including a firstlight-transmitting substrate having at least two parallel major surfacesand two opposite edges; an input aperture; an output aperture positionednext to one of the major surfaces of the substrate; an eye-motion boxhaving an aperture; a first intermediate element having at least twosurfaces positioned outside of the substrate for coupling light waves,having a field-of view, into the substrate through the input aperture; afirst flat reflecting surface, having an active area located between thetwo major surfaces of the light-transmitting substrate, for reflectingthe coupled-in light waves to effect total internal reflection from themajor surfaces of the substrate; a second flat reflecting surfaceparallel to the first flat reflecting surface, having an active area andbeing located between the two major surfaces of the light-transmittingsubstrate, for coupling light waves out of the substrate, and aredirecting optical element having at least two surfaces positionedoutside of the substrate for redirecting light waves coupled-out fromthe substrate through the output aperture, into the eye-motion-box,wherein the input aperture is substantially smaller than the outputaperture, the active area of the first reflecting surface is similar tothe active area of the second reflecting surface, and each of thecoupled light waves covers the entire aperture of the eye-motion-box.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with certain preferredembodiments, with reference to the following illustrative figures sothat it may be more fully understood.

With specific reference to the figures in detail, it is stressed thatthe particulars shown are by way of example and for the purpose ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented to provide what is believed to be themost useful and readily understood description of the principles andconceptual aspects of the invention. In this regard, no attempt is madeto show structural details of the invention in more detail than isnecessary for a fundamental understanding of the invention. Thedescription taken with the drawings are to serve as direction to thoseskilled in the art as to how the several forms of the invention may beembodied in practice.

In the drawings:

FIG. 1 is a side view of a prior art exemplary light-transmittingsubstrate;

FIG. 2 is a side view of another prior art exemplary light-transmittingsubstrate;

FIGS. 3A and 3B illustrate desired reflectance and transmittancecharacteristics of selectively reflecting surfaces, used in a prior artexemplary light-transmitting substrate, for two ranges of incidentangles;

FIG. 4 illustrates a reflectance curve as a function of the incidentangle for an exemplary dielectric coating;

FIG. 5 is a schematic sectional view of a prior-art light-transmittingsubstrate, wherein the coupling-in, as well as the coupling-outelements, are diffractive optical elements;

FIGS. 6A, 6B and 6C illustrate sectional views of a prior-arttransparent substrate having coupling-in and coupling-out surfaces, anda partially reflecting redirecting element;

FIGS. 7 schematically illustrates the active parts of the coupling-outsurface, according to the viewing angle and the EMB of the system; FIGS.8A, 8B, 8C and 8D schematically illustrate the active parts of thecoupling-in surface, according to the viewing angle and the EMB of thesystem;

FIGS. 9A, 9B, 9C and 9D are schematic sectional views ofsubstrate-guided embodiments having a single coupling-out element, anintermediate prism, and an input aperture substantially smaller than theoutput aperture, according to the present invention;

FIG. 10 is a graph illustrating the reflection of incident light waveson an interface plane, for three different wavelengths, as a function ofthe incident angle, according to the present invention;

FIG. 11 is a graph illustrating the incident angle on an interface planeof two different light waves, and the critical angle of the interfaceplane, as a function of the wavelength, according to the presentinvention;

FIGS. 12A, 12B and 12C are graphs illustrating the reflection of theincident light waves on an interface plane for three differentwavelengths, as a function of the incident angle, and the incidentangles of two specific light waves, according to the present invention;

FIGS. 13A, 13B, 13C and 13D are schematic sectional views of othersubstrate-guided embodiments having a single coupling-out element, anintermediate prism, and an input aperture substantially smaller than theoutput aperture, according to the present invention;

FIGS. 14A, 14B, 14C and 14D are schematic sectional views of yet othersubstrate-guided embodiments having a single coupling-out element, anintermediate prism, and an input aperture substantially smaller than theoutput aperture, according to the present invention;

FIG. 15 is a graph illustrating the reflection of incident light waveson the coupling-in surface, for three different wavelengths, as afunction of the incident angle, according to the present invention;

FIG. 16 is a graph illustrating the incident angle on the coupling-insurface of two different light waves, and the critical angle of thecoupling-in surface, as a function of the wavelength, according to thepresent invention;

FIGS. 17A, 17B and 17C are graphs illustrating the reflection of theincident light waves on the coupling-in surface for three differentwavelengths, as a function of the incident angle, and the incidentangles of two specific light waves, according to the present invention;

FIGS. 18A, 18B, 18C and 18D are schematic sectional views ofsubstrate-guided embodiments having a single coupling-out element, twointermediate prisms, and an input aperture substantially smaller thanthe output aperture, according to the present invention;

FIGS. 19A, 19B, 19C and 19D are other schematic sectional views ofsubstrate-guided embodiments having a single coupling-out element, twointermediate prisms, and an input aperture substantially smaller thanthe output aperture, according to the present invention;

FIGS. 20A, 20B, 20C and 20D are yet other schematic sectional views ofsubstrate-guided embodiments having a single coupling-out element, twointermediate prisms, and an input aperture substantially smaller thanthe output aperture, according to the present invention;

FIG. 21 is a schematic sectional view of substrate-guided embodimenthaving two adjacent substrates with different inclination angles of thecoupling-in surfaces, according to the present invention;

FIGS. 22A, 22B, 22C and 22D are schematic sectional views of a singlelight wave coupled inside a substrate-guided embodiment having twoadjacent substrates, according to the present invention;

FIGS. 23A, 23B, 23C and 23D are schematic sectional views of anotherlight wave coupled inside a substrate-guided embodiment having twoadjacent substrates, according to the present invention;

FIGS. 24A, 24B, and 24C are schematic sectional views of yet anotherlight wave coupled inside a substrate-guided embodiment having twoadjacent substrates, according to the present invention;

FIG. 25 is a schematic sectional view of three different light wavescoupled inside a substrate-guided embodiment having two adjacentsubstrates, an intermediate prism, and an input aperture substantiallysmaller than the output aperture, according to the present invention;

FIGS. 26A and 26B are schematic sectional views of substrate-guidedembodiments wherein undesired light waves reach the EMB of the system;

FIG. 27 is a schematic sectional view of substrate-guided embodimenthaving an array of absorptive surfaces for eliminating the totalinternal reflection from an external surface, according to the presentinvention;

FIGS. 28A, 28B, 28C, 28D, 28E and 28F are diagrams illustrating a methodfor fabricating a plate having an array of absorptive surfaces,according to the present invention;

FIGS. 29A and 29B are schematic sectional views of substrate-guidedembodiments, wherein the undesired stray rays are absorbed inside a thinplate, according to the present invention;

FIG. 30 is a diagram illustrating a method to expand the output aperturealong two axes utilizing a double substrate configuration, according tothe present invention, and

FIGS. 31A and 31B are other schematic sectional views ofsubstrate-guided embodiments using a reflective lens as a collimatingelement for polarized and unpolarized display sources.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a sectional view of a prior art light-transmittingsubstrate, wherein a first reflecting surface 16 is illuminated by acollimated light wave 12 emanating from a display source 4 andcollimated by a lens 6 located between the source 4 and a substrate 20of the device. The reflecting surface 16 reflects the incident lightfrom the source 4 such that the light wave is trapped inside the planarsubstrate 20, by total internal reflection. After several reflectionsoff the major surfaces 26, 27 of the substrate 20, the trapped lightwaves reach a partially reflective element 22, which couple the lightout of the substrate into the eye 24, having a pupil 25, of a viewer.Herein, the input aperture 17 of the substrate 20 is defined as theaperture through which the input light waves enter the substrate, andthe output aperture 18 of the substrate is defined as the aperturethrough which the trapped light waves exit the substrate. In the case ofthe substrate illustrated in FIG. 1, both the input and the outputapertures coincide with the lower surface 26. Other configurations areenvisioned, however, in which the input and the image light waves fromthe displace source 4 are located on opposite sides of the substrate, oron one of the edges of the substrate. As illustrated, the active areasof the input and the output apertures, which are approximately theprojections of the coupling-in 16 and the coupling-out 22 elements onthe major surface 26, respectively, are similar to each other.

In HMD systems it is required that the entire area of the EMB isilluminated by all the light waves emerging from the display source, toenable the viewer's eye looking at the entire FOV of the projected imagesimultaneously. As a result, the output aperture of the system should beextended accordingly. On the other hand, it is required that the opticalmodule should be light and compact. Since the lateral extent of thecollimating lens 6 is determined by the lateral dimension of the inputaperture of the substrate, it is desired that the input aperture shouldbe as small as possible. In systems such as those illustrated in FIG. 1,wherein the lateral dimensions of the input aperture are similar to thatof the output aperture, there is an inherent contradiction between thesetwo requirements. Most of the systems based on this optical architecturesuffer from small EMB and small achievable FOV, as well as from a largeand cumbersome imaging module.

An embodiment which solves this problem, at least partially, isillustrated in FIG. 2, wherein the element which couples-out the lightwaves from the substrate is an array of partially reflecting surfaces 22a, 22 b etc. The output aperture of this configuration can be extendedby increasing the number of partially reflecting surfaces embeddedinside the substrate 20. It is thus possible to design and construct anoptical module having a small input aperture, as well as a large outputaperture. As can be seen, the trapped rays arrive at the reflectingsurfaces from two distinct directions 28, 30. In this particularembodiment, the trapped rays arrive at the partially reflecting surface22 a from one of these directions 28 after an even number of reflectionsfrom the substrate major surfaces 26 and 27, wherein the incident anglebetween the trapped ray and the normal to the reflecting surface isβ_(ref).

The trapped rays arrive at the partially reflecting surface 22 b fromthe second direction 30 after an odd number of reflections from thesubstrate surfaces 26 and 27, wherein the incident angle between thetrapped ray and the normal to the reflecting surface is β_(ref).

As further illustrated in FIG. 2, for each reflecting surface, each rayfirst arrives at the surface from the direction 30, wherein some of therays again impinge on the surface from direction 28. In order to preventundesired reflections and ghost images, it is important that thereflectance be negligible for the rays that impinge on the surfacehaving the second direction 28.

A solution for this requirement that exploits the angular sensitivity ofthin film coatings, was previously proposed in the Publications referredto hereinabove. The desired discrimination between the two incidentdirections can be achieved if one angle is significantly smaller thanthe other one. It is possible to provide a coating with very lowreflectance at high incident angles, and a high reflectance for lowincident angles. This property can be exploited to prevent undesiredreflections and ghost images by eliminating the reflectance in one ofthe two directions.

Referring now specifically to FIGS. 3A and 3B, these figures illustratedesired reflectance behavior of partially reflecting surfaces 34. Whilethe ray 32 (FIG. 3A), having an off-axis angle of β_(ref), is partiallyreflected and coupled out of the substrate 20, the ray 36 (FIG. 3B),which arrives at an off-axis angle of β′_(ref) to the reflectingsurfaces 34, is transmitted through the reflecting surfaces 34, withoutany notable reflection.

FIG. 4 illustrates the reflectance curve of a typical partiallyreflecting surface of this specific system, as a function of theincident angle for S-polarized light with the wavelength λ=550 nm. For afull-color display, similar reflectance curves should be achieved forall the other wavelengths in the relevant visible spectrum, which isusually, for most display sources, between 430 nm and 660 nm. There aretwo significant regions in this graph: between 65° and 85°, where thereflectance is very low, and between 10° and 40°, where the reflectanceincreases monotonically with increasing incident angles. As can be seenin FIGS. 3 and 4, the requested reflectance behavior of the partiallyreflective surfaces 22 of the embodiment illustrated in FIG. 2, is notconventional. Furthermore, to keep the low reflectance at the higherangular region, the reflectance at the lower angular region cannot behigher than 20%-30%. Furthermore, to achieve a uniform brightness overthe entire FOV, it is required that the reflectance of partiallyreflecting surfaces will be increased gradually toward the edge of thesubstrate, and hence, the maximum achievable efficiency is comparativelylow an and usually cannot be more than 10%.

Another approach to couple light waves into and out from a light-guidedoptical element is by using diffractive elements. As illustrated in FIG.5, the light rays 34 and 36 are coupled into the transparent substrate20 by a diffractive element 48, and after several total internalreflections from the external surfaces of the substrate, the light raysare coupled-out from the substrate by a second diffractive element 50.As illustrated, ray 34 is coupled-out at least twice at two differentpoints 52 and 54 on element 54. Consequently, to achieve uniform outputlight waves, the diffraction efficiency of element 50 should beincreased gradually along the 4 axis. As a result, the overallefficiency of the optical system is even lower than that of the systemillustrated in FIG. 2, and it is usually not more than a few percent.That is to say, in the embodiments illustrated in FIGS. 2 and 5, theoutput aperture is extended to be much larger than the input aperture atthe cost of significantly reducing the brightness efficiency of theoptical module, as well as complicating the fabricating process of thesubstrate.

FIGS. 6A and 6B illustrate embodiments for overcoming theabove-described problem. Instead of using a single element (22 in FIG.2, or 50 in FIG. 5), which performs the dual function of coupling thelight waves out of the substrate 20, as well as directing the lightwaves into the user's eye 24, the requested function is divided into twodifferent elements; namely, one element, which is embedded inside thesubstrate couples the light waves out of the substrate, while a secondconventional partially reflecting element, which is located out of thesubstrate, redirects the light waves into the viewer's eye. Asillustrated in FIG. 6A, two rays 63 (dashed lines) from a plane lightwave emanating from a display source and collimated by a lens (notshown) enter a light transparent substrate 64, having two parallel majorsurfaces 70 and 72, through the input aperture 86, at an incident angleof α_(in) ⁽⁰⁾ with respect to the major surfaces 70, 72 of thesubstrate. The rays impinge on the reflecting surface 65, which isinclined at an angle α_(sur1) to the major surfaces of the substrate.The reflecting surface 65 reflects the incident light rays such that thelight rays are trapped inside a planar substrate 64 by total internalreflection from the major surfaces. In order to differentiate betweenthe various “propagation orders” of the trapped light waves, asuperscript (i) will denote the order i. The input light waves whichimpinge on the substrate in the zero order are denoted by thesuperscript (0). After each reflection from the coupling-in reflectingsurface the order of the trapped ray is increased by one from (i) to(i+1). The off-axis angle α_(in) ⁽¹⁾ between the trapped ray of thefirst order and the normal to the major surfaces 70, 72 is

$\begin{matrix}{\alpha_{in}^{(1)} = {\alpha_{in}^{(0)} + {2 \cdot {\alpha_{{sur}1}.}}}} & (1)\end{matrix}$

After several reflections off the surfaces of the substrate, the trappedlight rays reach a second flat reflecting surface 67, which couples thelight rays out of the substrate. Assuming that surface 67 is inclined atthe same angle to the major surfaces as the first surface 65, that is tosay, surfaces 65 and 67 are parallel and α_(sur2)=α_(sur1), then theangle α_(out) between the coupled-out rays and the normal to thesubstrate plane is

$\begin{matrix}{\alpha_{out} = {{\alpha_{in}^{(1)} - {2 \cdot \alpha_{{sur}2}}} = {{\alpha_{in}^{(1)} - {2 \cdot \alpha_{{sur}1}}} = {\alpha_{in}^{(0)}.}}}} & (2)\end{matrix}$

Hence, the coupled-out light rays are inclined to the substrate at thesame angle as the incident light rays. So far, the coupled-in lightwaves behave similarly to the light waves illustrated in FIG. 1. FIG.6A, however, illustrates a different behavior wherein two light rays 68(dashed-dotted lines), having the same incident angle of α_(in) ⁽⁰⁾ asrays 63, impinge on the right side of the reflecting surface 65. Aftertwo reflections from surface 65, the light waves are coupled inside thesubstrate 64 by a total internal reflection, and the off-axis angle ofthe trapped rays inside the substrate is now

$\begin{matrix}{\alpha_{in}^{(2)} = {{\alpha_{in}^{(1)} + {2 \cdot \alpha_{{sur}1}}} = {\alpha_{in}^{(0)} + {4 \cdot {\alpha_{{sur}1}.}}}}} & (3)\end{matrix}$

After several reflections off the major surfaces of the substrate, thetrapped light rays reach the second reflecting surface 67. The lightrays 68 are reflected twice from the coupling-out surface 67 and arecoupled out from the substrate at the same off-axis angle α_(out) as theother two rays 63 which are reflected only once from surfaces 65 and 67,which is also the same incident input angle of these four rays on thesubstrate major planes. Although all the four rays impinge and arecoupled-out of the substrate at the same off-axis angle, there is asubstantial difference between them: the two light rays 68 whichincident on the right side of the reflecting surface 65 are closer tothe right edge 66 of substrate 64, are reflected twice from surfaces 65and 67, and are coupled-out from the substrate at the left side ofsurface 67, which is closer to the opposite left edge 69 of thesubstrate. On the other hand, the two light rays 63 which incident onthe left side of the reflecting surface 65, are closer to the center ofsubstrate 64, and are reflected once from surfaces 65 and 67, and arecoupled-out from the substrate at the right side of surface 67, which iscloser to the center of the substrate.

As further illustrated in FIGS. 6A and 6B, the inclination angle α_(out)of the image can be adjusted by adding a partially reflecting surface 79which is inclined at an angle of α_(red) to the surface 72 of thesubstrate. As shown, the image is reflected and rotated such that itpasses again through the substrate substantially normal to thesubstrate's major surfaces and reaches the viewer's eye 24 through theoutput aperture 89 of the substrate. To minimize distortion andchromatic aberrations, it is preferred to embed surface 79 in aredirecting prism 80, and to complete the shape of the substrate 64 witha second prism 82, both of them fabricated of the same material which,should not necessarily be similar to that of prism 80. In order tominimize the thickness of the system, it is possible, as illustrated inFIG. 6B, to replace the single reflecting surface 79 with an array ofparallel partially reflecting surfaces 79 a, 79 b, etc., where thenumber of the partially reflecting surfaces can be determined accordingto the requirements of the system. Another way to redirect thecoupled-out light waves into the viewer's eye is to use a flatmeta-surface that is structured with subwavelength-scaled patterns.

In the illustrated embodiments herein, it is assumed that light waveshaving only the first and the second orders of axis-axis angles,propagate inside the substrate. There are systems, however, havingcomparatively small inclination angle α_(sur1) of the coupling-in andthe coupling-out surfaces, where even the third and the fourth orderscan be utilized. As illustrated in FIG. 6C, an input ray 71 impinges onsubstrate 64 having an off-axis angle α_(in) ⁽⁰⁾. After threereflections from surface 65 at points 75 a, 75 b and 75 c, this ray iscoupled inside the substrate and propagates inside it having the thirdorder off-axis angle of α_(in) ⁽³⁾. After a few reflections from themajor surfaces of the substrate 64, the ray 71 impinges on surface 67.After three reflections from the surface at points 77 a, 77 b and 77 cit is coupled out from the substrate 64 having an off-axis angle α_(in)⁽⁰⁾. The light ray 71 is then reflected by surface 79 a, substantiallynormal to the substrate's major surface into the viewer's eye 24. As arule, for systems having a few coupling-in orders, the lower order willbe coupled into and from the substrate at the parts of the reflectingsurfaces closer the substrate's edges, the higher order will be coupledat the parts of the reflecting surfaces' closer to the center of thesubstrate, while the middle order will be coupled from the central partsof the coupling-in and the coupling-out surfaces.

There are two contradicting requirements from the coupling-out surface67. On the one hand, the first three order images F⁽¹⁾, F⁽²⁾ and F⁽³⁾should be reflected from that plane, while on the second hand, thezero-order image F⁽⁰⁾ from the substrate 64 should substantially passthrough it, after being reflected from surface 79, with no significantreflections. In addition, for see-through systems, the transparency ofthe optical system for substantially normal incident light ray 83 fromthe external scene should be as high as possible. A way to achieve thisis to use an air gap in surface 67. For achieving a rigid system, it ispreferred, however, to apply an optical adhesive in surface 67, in orderto cement the substrate 64 with prism 82 using an optical adhesivehaving a refractive index, which is substantially smaller than that ofthe substrate. There are situations, however, wherein the requiredrefractive index of the optical adhesive, which yields the necessarytotal internal reflection effect for the entire coupled FOV, is verylow—in the order of 1.31-1.35. There are optical adhesives that arecommercially available and have the required refractive index. Still,usually their adhesion strength is not good enough, and their resistanceto extreme environmental conditions is also insufficient for militaryand professional applications. An alternative solution is to apply athin film of dielectric material on surface 67, using a spin coatingprocedure. The refractive index of the applied coating material issubstantially smaller than that of the substrate, and should have theappropriate value, which yields the required total internal reflectionfrom surface 67 for the entire FOV. Substrate 64 can be cemented now toprism 82 using an optical adhesive having the required adhesion strengthand resistance to environmental conditions while its exact refractiveindex can have any reasonable value.

In any of the proposed approaches to minimize the Fresnel reflections ofthe transmitted light waves from the coupling-out surface 67, it ispreferred to apply a suitable anti-reflective (AR) coating to thissurface. In that case, the overall efficiency of light waves which passthrough substrate can be very high, namely, the reflectance of surface67 when coupling the light waves out of the substrate, is 100% as aresult of the total internal reflection from that surface, while thetransmission of that surface to the reflected light waves from surface79, as well as for the light rays from the external scene, is also closeto 100% as a result of the AR coating. Similarly, it is preferred tocement prism 80 to the lower surface 72 of substrate 64, defining aninterface plane 81, using an optical adhesive having a refractive index,which is substantially smaller than that of the substrate, wherein anappropriate AR coating is applied to this interface plane. Here again,the total internal reflection from surface 72 can be achieved byapplying an appropriate material using spin coating on surface 72 andusing a conventional optical adhesive to cement prism 80 to surface 72.Consequently, the brightness of light waves, which are coupled out bysurface 67 from the substrate, is similar to the brightness of the inputlight waves before being coupled into the substrate by surface 65, andthe only place where their brightness is attenuated is by the partialreflection from surface 79. As a result, the brightness efficiency ofthe embodiment illustrated in FIGS. 6A-6C can be higher by an order ofmagnitude than the efficiency of the configurations illustrated in FIGS.2 and 5.

As explained above with regard to FIG. 6A, in see-through systems suchas HMDs for augmented reality (AR) applications, wherein the viewershould see the external scene through the substrate, the partiallyreflecting surfaces 79 should be at least partially transparent toenable the external light rays 63 and 68 passing through the substrateand reaching the viewer's eye 24. Since surfaces 79 are only partiallyreflective, only part of the coupled light waves 63 and 68 is reflectedby surfaces 79 and reaches the viewer's eye, while another part of thelight waves 84 passes through surfaces 79, coupled out from the prism 80and do not reach the viewer's eye. Similarly, since surfaces 79 are onlypartially transmissive, only part of the external light rays 83 passesthrough surfaces 79 and reaches the viewer's eye, while another part ofthe light rays 85 is reflected from surfaces 79, coupled out from theprism 80 and does not reach the viewer's eye as well. Naturally, theefficiency of the projected image can be increased on account of theexternal scene, and vice-versa, namely, by increasing the reflectivityof the partially surfaces 79 the brightness of the coupled rays 63 and68 is increased. Consequently, however, the transmissivity of surfaces79 is decreased, and hence, the brightness of the external image 83 isreduced accordingly.

In contradiction to the embodiments illustrated in FIGS. 1-5, thesurface 79 that reflects the coupled-out light from the substrate to theviewer's eye and, at the same time, transmits the external rays, is aconventional partially reflecting mirror without any special orcomplicated characteristics as surfaces 22 and 50 of the embodimentsillustrated in FIGS. 2 and 5, respectively. As a result, it is possibleto dynamically control the reflectivity (and consequently, thetransmissivity) of the partially reflective surfaces 79, according tothe external lighting conditions and the specific image which isprojected to the viewer's eye. One way to control the reflectivity ofsurfaces 79 is by using an electrically switchable transreflectivemirror, which is a solid-state thin film device made from a specialliquid crystal material, and which can be rapidly switched between purereflection, partial-reflection, and total transparent states. Anothermethod to achieve a switchable element 79 is by forming it as a dynamicmetasurface. The required state of the switchable mirror can be seteither manually by the user, or automatically, by using a photometerwhich controls the reflectivity of the mirror according to the externalbrightness. This feature can be useful for conditions in which theprojected image is properly combined with the external image, but thebrightness of the external scene is comparatively high, and hence, itshould be mostly blocked from dazzling the viewer and from interferingwith the projected image. On the other hand, the efficiency of theprojected image should be high enough to achieve a reasonable contrast.Therefore, the dynamic surface 79 can be switched into a primaryreflection state, namely, the reflection of the switchable mirror ismuch higher than its transmission. As a result, the coupled out lightrays 63 and 68 from the substrate are mainly reflected from surface 79to the viewer's eye, and the overall efficiency of the optical systemcan be more than 90% while the bright external scene can still be seenproperly. Consequently, the potential brightness efficiency of theembodiment illustrated in FIGS. 6A-6C can be higher by more than anorder of magnitude than the efficiency of the configurations illustratedin FIGS. 2 and 5.

As seen in FIGS. 6A-6C, the aperture of the coupling-in surface 65 issimilar to that of the coupling-out surface 67. Subsequently, the activearea of the input aperture 86 is similar to that of the output aperture89. As a result, although the potential brightness efficiency of theembodiment illustrated in FIGS. 6A-6C can be very high, it still suffersfrom the problem of similar input and output apertures. Therefore, anappropriate way should be found to reduce the input aperture for a givenoutput aperture, or alternatively, to increase the output aperture for agiven input aperture. In order to achieve this, the fact that the lightwaves coupled out from the substrate do not have to illuminate theentire active area of the coupling-out surface, is utilized.

FIG. 7 demonstrates the rays that should impinge on the output apertureof surface 89, in order to illuminate the EMB 100, including the twomarginal and the central light waves of the image which are coupled outfrom the substrate and re-directed into the viewer's eye 24. As shown,the light waves 107R, 107M, and 107L, having the zero order off-axisangles α_(in) ⁽⁰⁾(max), α_(in) ⁽⁰⁾(mid) and α_(in) ⁽⁰⁾(min), which arethe minimal, central and maximal angles in the FOV respectively,illuminate only the parts 67R, 67M and 67L of the coupling-outreflecting surface 67, respectively, and are reflected by surface 89into to EMB 100. A method can thus be determined, wherein the inputaperture of the substrate will be significantly reduced, so that thecoupled-in light waves will illuminate only the required respective partof surface 67, and hence, the original brightness will be preserved.

FIGS. 8A-8D illustrate the tracing-back of the three light waves fromthe EMB toward the input aperture 86 of the substrate 64. As shown, thelight wave 107L (dashed-dotted lines, FIG. 8A) impinges on the rightpart of surface 65, trapped inside the substrate having an off-axisangle α_(in) ⁽³⁾ after three reflection from surface 65, and iscoupled-out from the substrate after three reflections from the surface67, wherein the third reflection which couples the light wave out of thesubstrate is at the left part of surface 67. The light wave 107M (dottedlines, FIG. 8B) impinges on the central part of surface 65, trappedinside the substrate having an off-axis angle α_(in) ⁽²⁾ after tworeflection from surface 65, and is coupled-out from the substrate aftertwo reflections from the surface 67, wherein the second reflection whichcouples the light wave out of the substrate is at the central part ofsurface 67. The light wave 107R (dashed lines, FIG. 8C) impinges on theleft part of surface 65, trapped inside the substrate having an off-axisangle α_(in) ⁽¹⁾ after one reflection from surface 65, and iscoupled-out from the substrate after one reflection from the right partof the surface 67. As illustrated in FIG. 8D, the lateral area of theinput aperture 86, which covers the incoming light waves over the entireFOV, is similar to that of the output aperture 89, and hence, in thisembodiment the target of reducing the input aperture 86 has not beenachieved.

It should be noted however, that although the incoming waves cover aconsiderably large input aperture, they impinge on the input aperture atan orientation opposite to that of a conventional optical system. Thatis to say, when tracing the light waves backwards from the inputaperture 86, instead of diverging away they are converging to becomecloser to each other. As a result, an intermediate prism can be added tothe optical system, which will enable the traced-back light waves to beconverged into a substantially smaller pupil than that of the inputaperture 86.

FIGS. 9A-9D illustrate the embodiment shown in FIGS. 8A-8D, wherein anintermediate prism 108 is attached to the substrate 64 at the inputaperture 86. The surface 110 of prism 108 can be optically attached tothe upper surface 70 of the substrate 64, defining an interface plane111. To minimize chromatic dispersion, the optical material of the prism108 should be similar to that of the redirecting prism 80. In addition,the light waves input surface 112 of prism 108 should be oriented suchthat the incoming waves 107R, 107M and 107L will impinge on surface 112at the same angles which they are coupled out from the substrate 64through the upper surface 70 toward the viewer's eye 24. Moreover,surface 112 should be located in a plane where the traced-back lightwaves are converged to a minimal aperture. As illustrated in FIG. 9D,all the incoming light waves incident on surface 112 inside a new inputaperture 86′ which is substantially smaller, by far more than a factorof two, than the original input aperture 86, as well as the outputaperture 89.

There are two contradicting requirements from the interface plane 111between the intermediate prism 108 and the substrate 64. On the onehand, the first three orders image F⁽¹⁾, F⁽²⁾ and F⁽³⁾ should bereflected from that plane, while the zero-order image) F⁽⁰⁾ entering thesubstrate 64 through the intermediate prism 111, should substantiallypass through it with no significant reflections Similarly, the interfaceplane 81 between the substrate 64 and the redirecting prism 80 should betransparent to the coupled-out light waves having the input angle ofα_(in) ⁽⁰⁾ after the last reflection from surface 67, and at the sametime highly reflective for the coupled light waves having the higherorder input angles of α_(in) ⁽¹⁾, α_(in) ⁽²⁾ and α_(in) ⁽³⁾. Inaddition, for see-through systems the transparency of the optical systemfor substantially normal incident light, through the interface plane 81,should be as high as possible. A preferred way of achieving it is toapply an optical adhesive to these interface planes, having a refractiveindex which is substantially smaller than that of the substrate, oralternatively, to apply a thin film having the required refractive indexon the interface plane 81 using a spin coating procedure. In addition,to minimize the Fresnel reflections of the transmitted light waves fromthe interface planes 81 and 111, it is preferred to apply a suitable ARcoating to these planes. In that case, the overall efficiency of lightwaves which interact with these planes can be very high. That is to say,the reflectance of plane 111 when coupling the light waves into thesubstrate is 100% as a result of the total internal reflection from thatsurface while the transmission of that surface to the incoming lightwaves is also close to 100% as a result of the AR coating. Similarly,the reflectance of the light waves coupled inside the substrate 64 fromsurface 81, is 100% as a result of the total internal reflection fromthat surface, while the transmission of that surface to the light wavescoupled-out from the substrate 64 into the redirecting prism 80, as wellas for the incoming light waves from the external scene, is also closeto 100% as a result of the AR coating.

For most of the relevant display systems, the two requirements should befulfilled over the entire relevant visible spectrum. Therefore, it isreasonable to assume that the Abbe numbers of the optical adhesive (oralternatively, the thin film which is applied by spin coating), which isadjacent to the interface surfaces, and the optical material of thesubstrate, should be similar to avoid undesired chromatic effects in theimage. To achieve the required total internal reflection phenomena, therefractive indices of the substrate and the adhesive (or the thin film)should be significantly different. As a result, it will be verydifficult to fulfill this requirement and usually the Abbe numbers ofthe adhesive (or the thin film) and the optical material will besubstantially different. The chromatic dispersion due to the variationbetween the Abbe numbers can be compensated, however, by choosing anoptical material for the coupling-in and the redirecting of prisms 108and 80, having an Abbe number which is different than that of thesubstrates 64. By a proper selection, the difference between the Abbenumbers can induce a chromatic dispersion having the same magnitude andan opposite direction. As a result, the two induced dispersions will bemutually compensated.

Another issue to consider is the maximum achievable FOV of the imagewhich is projected into the viewer's eye. In most of thesubstrate-guided based HMD technologies, either reflective ordiffractive, the light waves are coupled out from the guiding substratesubstantially normal to the major surfaces of the substrate.Consequently, due to the Snell refraction from the substrate theexternal FOV of the image is:

$\begin{matrix}{{\left. F^{({out})} \right.\sim F^{({in})}} \cdot v_{s}} & (4)\end{matrix}$

wherein the FOV inside the substrate is F^((in)) and the refractiveindex of the substrate is □_(S). In addition, the orders of the lightwaves which are coupled inside the substrate should be strictlyseparated, namely,

$\begin{matrix}{\alpha_{\min}^{(1)} = {{\alpha_{\min}^{(0)} + {2 \cdot \alpha_{{sur}1}}} > {\alpha_{\max}^{(0)}.}}} & (5)\end{matrix}$

In addition, to ensure the transmission of the entire zero-order throughthe interface planes 81 and 111, and the reflection of the entire firstorder from these planes, the following constraint

$\begin{matrix}{{{\alpha_{cr} > \alpha_{\max}^{(0)}};}{\alpha_{cr} < \alpha_{\min}^{(1)}}} & (6)\end{matrix}$

must be fulfilled, wherein α_(cr) is the critical angle for theinterface planes. Therefore, the internal FOV is limited by theconstraint

$\begin{matrix}{F^{({in})} = {{\alpha_{\max}^{(0)} - \alpha_{\min}^{(0)}} < {2 \cdot {\alpha_{{sur}1}.}}}} & (7)\end{matrix}$

wherein usually a margin in the order of one degree should be keptbetween α_(max) ⁽⁰⁾ and α_(min) ⁽¹⁾ to confirm the separation betweenthe two orders. The limitation of Eq. (4) yields for systems wherein therefraction indices of the substrate, the coupling-in and thecoupling-out elements are equal.

The fact that the optical light waves enter the substrate 64 from theintermediate prism 108 at highly oblique angles can be used to improvethe above limitation. As illustrated in FIGS. 9A-9D, the intermediate108 and the redirecting 80 prisms are fabricated from the same opticalmaterial having refractive index which have the following opticalcharacteristic

$\begin{matrix}{v_{p} < v_{s}} & (8)\end{matrix}$

wherein α_(s) and α_(p) is the refractive index of the prisms 108 and80. In addition, A_(p), A_(s), the Abbe numbers of the prisms and thesubstrates respectively, are chosen to compensate for the chromaticdispersion induced by the dissimilarity between the Abbe numbers of thesubstrate and the optical adhesive (or the thin film) as explainedabove.

As a result of the dissimilarities between the optical material of thesubstrates 64 and that of the intermediate 108 and the redirectingprisms 80, and the high obliquity that rays 107R, 107M and 107L incidenton the interface surfaces 111 and 81, the light waves experiencesubstantial refraction when passing through the interface surfaces.Since prisms 108 and 80 have the same optical characteristics, therefractions at surfaces 111 and 81 for each passing light wave will havethe same magnitude and the opposite directions respectively, andtherefore, they will be mutually compensated. The angular deviationbetween two different light rays inside the prisms as a function of thedeviation inside the substrates can be calculated according to theapproximated equation

$\begin{matrix}{{{\left. {\Delta\alpha}_{p} \right.\sim\frac{v_{s}}{v_{p}}} \cdot \frac{{cos\alpha}_{s}}{{cos\alpha}_{p}} \cdot {\Delta\alpha}_{s}},} & (9)\end{matrix}$

wherein α_(s) and α_(p) are the off-axis angles inside the substrate andthe prisms, respectively. Similarly, the angular deviation between therays in the air is

$\begin{matrix}{{\left. {\Delta\alpha}_{out} \right.\sim v_{p}} \cdot {{\Delta\alpha}_{p}.}} & (10)\end{matrix}$

Consequently, the ratio between the angular deviation in the air andinside the substrate 64 is

$\begin{matrix}{{{\left. {\Delta\alpha}_{out} \right.\sim v_{s}} \cdot \frac{{cos\alpha}_{s}}{{cos\alpha}_{p}} \cdot {\Delta\alpha}_{s}},} & (11) \\{or} & \; \\{{\left. F^{({out})} \right.\sim F^{({in})}} \cdot v_{s} \cdot {\frac{{cos\alpha}_{s}}{{cos\alpha}_{p}}.}} & (12)\end{matrix}$

That is to say, by modifying the optical material of the prisms 108 and80, it is possible to increase the FOV of the system in the air by afactor of

$\frac{{cos\alpha}_{s}}{{cos\alpha}_{p}}.$

The implementation of the embodiment shown in FIGS. 9A-9D is illustratedherein with an optical system having the following nominal parameters:

$\begin{matrix}{{{{{\alpha_{{sur}1} = {\alpha_{{sur}2} = {7{^\circ}}}};}F_{air}^{(0)} = \left\{ {{{- 20}{^\circ}},{20{^\circ}}} \right\}};}{F_{p}^{(0)} = \left\{ {{48.6{^\circ}},74.6} \right\}}{{F_{s}^{(0)} = \left\{ {{36.6{^\circ}},49.7} \right\}};}{{F_{s}^{(1)} = \left\{ {{50.6{^\circ}},63.7} \right\}};}{{F_{s}^{(2)} = \left\{ {{64.6{^\circ}},77.7} \right\}},{{F_{s}^{(3)} = \left\{ {{78.6{^\circ}},91.7} \right\}};}}{{F_{{sur}1}^{(0)} = \left\{ {{43.6{^\circ}},56.7} \right\}};}{{F_{{sur}1}^{(1)} = \left\{ {{57.6{^\circ}},70.7} \right\}},}} & (13)\end{matrix}$

wherein the light waves are unpolarized, the optical material of thesubstrate 64 is Ohara S-LAH88 having a refractive index of v_(d)=1.917and Abbe number of A_(d)=31.6, the optical material of the prisms 81 and111 is Schott N-BK7 having a refractive index of v_(d)=1.516 and Abbenumber of A_(d)=65.5, the optical adhesive which are adjacent tosurfaces 81 and 111 in FIGS. 9A-9D is NOA 148, having refractive indexof v_(d)=1.48 and Abbe number of A_(d)=48. As shown, the FOV is 40° inthe air, 26° inside the prisms 111 and 81 and 13° inside the substrate64. That is, the FOV in the air is expanded by a factor of more thanthree compared to the FOV inside the substrate, even though therefractive index of the substrate is less than 2. The maximal angle inthe third order is bigger than 90°, and hence, it has “illegal”propagation direction. As shown in FIGS. 9A-9C, however, the third orderis active only for the light waves in the lower region of the FOV. Thelight waves in the upper region of the FOV are coupled inside thesubstrate after a single reflection from the coupling-in surface 65, andhence, they are propagating only in the first propagation order, andthis contradiction is avoided.

FIG. 10 illustrates the reflection curve of an AR coating which isapplied at the interface surfaces 81 and 111. As a result of thechromatic dispersion due to the variation between the Abbe numbers ofthe substrate 64 and the prisms 81 and 111, the critical angle dependsstrongly on the wavelength. Therefore, the condition

$\begin{matrix}{{\alpha_{\min}^{(1)} > {51{^\circ}}}{and}{\alpha_{\max}^{(0)} < {49{^\circ}}}} & (1)\end{matrix}$

should assumingly be fulfilled over the entire relevant visible spectrumto satisfy the condition of Eq. (5). That is to say, the FOV inside thesubstrate should be reduced to 12°, and consequently, the FOV in the airwill be reduced to 36°.

The high dispersion of the light waves which enter the substrate 64through the intermediate prism 111 causes the spatial separation of eachincoming white light wave into components of different wavelengths. Forexample, the marginal light wave 107R which has an off-axis angle of−20° for the entire visible spectrum is split into the propagationdirections of 36.2°, 36.6° and 36.8° for the zero-order light waveshaving the wavelengths of 450 nm, 550 nm and 650 nm respectively. Theexact values of the parameters given in Eq. (13) for three differentwavelengths are

$\begin{matrix}{{{{F_{sb}^{(0)} = \left\{ {{36.2{^\circ}},{49.1{^\circ}}} \right\}};}{{F_{sg}^{(0)} = \left\{ {{36.6{^\circ}},49.7} \right\}};}{{F_{sr}^{(0)} = \left\{ {{36.8{^\circ}},50.1} \right\}},{{F_{sb}^{(1)} = \left\{ {{50.2{^\circ}},63.1} \right\}};}}}{{F_{sg}^{(1)} = \left\{ {{50.6{^\circ}},63.7} \right\}};}{{F_{sr}^{(1)} = \left\{ {{50.8{^\circ}},64.1} \right\}},{{F_{sb}^{(2)} = \left\{ {{64.2{^\circ}},77.1} \right\}};}}{{F_{sg}^{(2)} = \left\{ {{64.6{^\circ}},77.7} \right\}};}{{F_{sr}^{(2)} = \left\{ {{64.8{^\circ}},78.1} \right\}},{{F_{sb}^{(3)} = \left\{ {{78.2{^\circ}},91.1} \right\}};}}{{F_{sg}^{(3)} = \left\{ {{78.6{^\circ}},91.7} \right\}};}{{F_{sr}^{(3)} = \left\{ {{78.8{^\circ}},92.1} \right\}},{{F_{surb}^{(0)} = \left\{ {{43.2{^\circ}},56.1} \right\}};}}{{F_{surg}^{(0)} = \left\{ {{43.6{^\circ}},56.7} \right\}};}{{F_{surr}^{(0)} = \left\{ {{43.8{^\circ}},{57.1{^\circ}}} \right\}},}} & (15)\end{matrix}$

wherein the subscripts sb, sg and sr denote the parameters of the lightwaves inside the substrate 64, having the wavelengths of 450 nm, 550 nmand 650 nm respectively, and the subscripts surb, surg and surr denotethe parameters of the incoming light waves impinging on the coupling-insurface 65, having the same wavelengths, respectively.

FIG. 11 illustrates the propagation directions α_(min) ⁽¹⁾ and α_(max)⁽⁰⁾, as well as the critical angle α_(cr), as a function of thewavelength for the entire relevant visible spectrum. As shown, for theentire spectrum the requirements given in Eqs. (5)-(6) are fulfilledwithout submitting to the limitation of Eq. (14), and the FOVs of atleast 13° in the substrate and 40° in the air are preserved.

FIGS. 12A-12C illustrate, for the wavelengths 450 nm, 550 nm, and 650 nmrespectively, the reflection curves of the AR coating which is appliedat the interface surfaces 81 and 111, wherein two vertical lines denotethe propagation directions α_(min) ⁽¹⁾ and α_(max) ⁽⁰⁾ on the graph foreach relevant wavelength. As shown, for all the wavelengths thereflection for the angle α_(min) ⁽¹⁾ is 100% as a result of the totalinternal reflection from the interface plane, while the transmission offor the angle α_(max) ⁽⁰⁾ is negligible as required.

In FIGS. 9A-9D there is illustrated an embodiment of an optical systemhaving a wide FOV of 40° along the propagation direction of the lightwaves inside the substrate 64, even though only a single coupling-outelement 67 is utilized. The incoming directions of the input lightwaves, however, are at highly oblique angles. In many applications it isrequired that the incoming light waves will impinge on the substratesubstantially normal to the major surfaces 70 and 72 of the substrate.FIGS. 13A-13C illustrate a configuration wherein the left marginal 107L,the central 107M and the right marginal 107R light waves, respectively,impinge on the substrate substantially normal to the lower surface 72.As shown, the light waves enter the substrate and pass through thecoupling-in surface 65. Since the incident angles of the input lightwaves are substantially small and an AR coating is applied at surface65, the reflectance of the light waves from this surface will benegligible. The light waves exiting the substrate 64, enter theintermediate prism 114 through its lower surface 116, which is attachedto the upper surface 70 of the substrate, are reflected from thereflective surface 118, and re-enter the substrate 64 through its uppersurface 70 at input angles of α_(in) ⁽⁰⁾. The light waves now impinge onthe coupling-in surface 65 having the incident angles of α_(in)⁽⁰⁾+α_(Sur1), which angles are higher than the critical angle, and arecoupled inside the substrate in a similar manner as illustrated above inrelation to FIGS. 9A-9D. As illustrated in FIG. 13D, the light waves inthe entire FOV incident on surface 72 inside a new input aperture 86′which is substantially smaller, by at least a factor of three, than theoriginal input aperture 86 as well as the output aperture 89. Here theinput aperture 86′ is not located adjacent to the intermediate prism114, but rather next to the lower major surface 72 of the substrate. Ingeneral, the optical system should be designed such that the inputaperture will be positioned in a convenient place for placing theexternal surface of the collimating module.

In the embodiment illustrated in FIGS. 13A-13D the light waves impingeon the substrate at surface 72 and the light waves exit the substrateinto the viewer's eye through the opposite surface 70, namely, theviewer's eye and the display source are positioned at opposite sides ofthe substrate. This configuration is preferable for top-downconfiguration, however, there are other arrangements such as aneyeglasses structure, wherein it is required that the viewer's eye andthe display source will be located at the same side of the substrate.

FIGS. 14A-14C illustrate a configuration wherein the left marginal 107L,the central 107M and the right marginal 107R light waves, respectively,impinge on the substrate substantially normal to the upper surface 70,at the same side of the viewer's eye. A lens 6 is added to the figure toillustrate the collimating of the light waves coming from the displaysource 4. As shown, the light waves enter the substrate and pass throughthe coupling-in surface 65 with no significant reflections. The lightwaves exit the substrate 64, enter the intermediate prism 120 throughits upper surface 124, which is attached to the lower surface 72 of thesubstrate, are reflected from the reflective surface 122, and enteragain the substrate 64 through its lower surface 72. The light wavesimpinge again on the coupling-in surface 65 having the incident anglesof α_(in) ⁽⁰⁾−α_(Sur1), which are lower than the critical angle, passthrough the surface 65, and are totally reflected from the upper surface70 of the substrate. The light waves impinge again on the coupling-insurface 65 now having the incident angles of α_(in) ⁽⁰⁾+α_(Sur1), whichare higher than the critical angle, and are coupled inside the substratein a similar manner as illustrated above in relation to FIGS. 13A-13C.As illustrated in FIG. 14D, the light waves in the entire FOV incidenton surface 70 inside a new input aperture 86′ located next to theexternal surface of the collimating lens 6, is substantially smallerthan the original input aperture 86.

Unlike the other configurations illustrated hereinbefore, in theembodiment described with regard to FIGS. 14A-14D, the light wavesimpinge on the coupling-in surface 65 three times. The first time, therequirement that the light waves will pass through surface 65 with nosignificant reflections, can be simply achieved by applying an ARcoating at surface 65. For the other two impingements, however, thereare two contradicting requirements from surface 65. On the one hand, thelight waves being incident on the surface at the third time, having theincident angles of α_(in) ⁽⁰⁾+α_(Sur1), should be reflected from thatsurface. On the other hand, the light waves being incident on thesurface at the second time, having the incident angles of α_(in)⁽⁰⁾−α_(Sur1), should substantially pass through it with no significantreflections. A preferred way to achieve this, as described above inrelation to the interface planes 81 and 111, is to apply an opticaladhesive to the coupling-in surface 65, or thin film by spin coating,having a refractive index, which is substantially smaller than that ofthe substrate. In addition, to minimize the Fresnel reflections of thelight waves which incident at the second time on surface 65, it isrequired to apply a suitable AR coating to these planes.

FIG. 15 illustrates the reflection curve of an AR coating which isapplied at the coupling-in surface 65 for a substrate having thefollowing parameters: the light waves are unpolarized, the opticalmaterial of the substrate 64 is Ohara S-LAH98 having a refractive indexof v_(d)=1.954 and Abbe number of A_(d)=32.32, the optical adhesivewhich is adjacent to surface 65 is NOA 1315, having refractive index ofv_(d)=1.315 and Abbe number of A_(d)=56. As a result of the chromaticdispersion due to the variation between the Abbe numbers of thesubstrate 64 and the optical adhesive, the critical angle dependsstrongly on the wavelength.

FIG. 16 illustrates the propagation directions α_(max) ⁽⁰⁾−α_(Sur1) andα_(min) ⁽⁰⁾+α_(Sur1), as well as the critical angle α_(cr), as afunction of the wavelength for the entire relevant visible spectrum. Asshown, for the entire spectrum there is a differentiation between theangular spectra of the second and the third impingements, and thespectra are located below and above the curve of the critical anglerespectively, as required.

FIGS. 17A-17C illustrate, for the wavelengths 450 nm, 550 nm, and 650 nmrespectively, the reflection curves of the AR coating which is appliedat the coupling-in surface 65, wherein two vertical lines denote thepropagation directions α_(max) ⁽⁰⁾−α_(Sur1) and α_(min) ⁽⁰⁾+α_(Sur1) onthe graph for each relevant wavelength. As shown for all thewavelengths, the reflection for the third impingement, having anincident angle of α_(in) ⁽⁰⁾+α_(Sur1), is 100% as a result of the totalinternal reflection from the interface plane, while the transmission forthe second impingement, having an incident angle of α_(in) ⁽⁰⁾−α_(Sur1),is negligible, as required.

While FIGS. 13A-13D and 14A-14D illustrate embodiments wherein the inputlight waves imping on the substrate substantially normal to the majorsurfaces, there are configurations wherein it is required that the inputlight waves will be oriented at oblique angles to the substrate. FIGS.18A-18D illustrate a modified version of the embodiment shown in FIGS.13A-13D. The light waves, which illuminate the substrate at a predefinedangle, enter the substrate through the surface 128 of a firstintermediate prism 126 which is attached to the lower surface 72 of thesubstrate 64, and pass through the coupling-in surface 65 with nosignificant reflections. The light waves then exit the substrate 64,enter a second intermediate prism 132 through its lower surface 136,which is attached to the upper surface 70 of the substrate, arereflected from the reflective surface 134, and re-enter the substrate 64through its upper surface 70. The light waves are reflected by thecoupling-in surface 65 and trapped inside the substrate in a similarmanner as illustrated above in relation to FIGS. 13A-13D.

FIGS. 19A-19D illustrate a modified version of the embodiment shown inFIGS. 14A-14D. The light waves, which illuminate the substrate 64 at apredefined angle, enter the substrate through the surface 140 of a firstintermediate prism 138 which is attached to the upper surface 70 of thesubstrate, and pass through the coupling-in surface 65 with nosignificant reflections. The light waves exiting the substrate 64, enterthe second intermediate prism 144 through its upper surface 148, whichis attached to the lower surface 72 of the substrate, are reflected fromthe reflective surface 146, and enter again the substrate 64 through itslower surface 72. Then, the light waves impinge again on the coupling-insurface 65 having incident angles of α_(in) ⁽⁰⁾−α_(Sur1), which arelower than the critical angle, pass through the surface 65, and aretotally reflected from the upper surface 70 of the substrate. The lightwaves impinge again on the coupling-in surface 65 having now theincident angles of α_(in) ⁽⁰⁾+α_(Sur1), which are higher than thecritical angle, and they are coupled inside the substrate in a similarmanner as illustrated above in relation to FIGS. 14A-14C.

FIGS. 14A-14D and 19A-19D illustrate embodiments which can be utilizedfor eyeglasses configurations. There are situations, however,particularly for consumer market applications wherein, for aestheticconsiderations, it is required that the folding prism, which is attachedto the front surface of the substrate 72, will be as small as possible.FIGS. 20A-20D illustrate modified versions of the embodiments shown inFIGS. 14A-14D and 19A-19D, wherein the light waves, which illuminate thesubstrate 64 at a predefined angle, enter the substrate through thesurface 228 of a first intermediate prism 226, which is attached to theupper surface 70, and pass through the coupling-in surface 65 with nosignificant reflections. The light waves exiting the substrate 64, enterthe second intermediate prism 220 through the upper surface 224, whichis attached to the lower surface 72 of the substrate, are reflected fromthe reflective surface 222, and enter again the substrate 64 through itslower surface 72. Here, however, the inclination angle of the reflectingsurface 222, compared to the major surface 72, is significantly smallerthan the inclination angle of surfaces 122 and 146 in the configurationsof FIGS. 14A-14D, and 19A-19D, respectively. As a result, the lightwaves impinge again on the coupling-in surface 65 having incident anglesof α_(in) ⁽⁰⁾−α_(Sur1)−ε, wherein ε is an angle which can be determinedaccording to design considerations, but is typically bigger than 5°.Now, even the maximal incident angle α_(max) ⁽⁰⁾−α_(Sur1)−ε isconsiderably lower than the critical angel, and hence a simpler ARcoating can be applied to surface 65. The light waves continue to passthrough the surface 65, enter again the first intermediate prism 226through its lower surface 230, which is attached to the upper surface 70of the substrate. The waves are then totally reflected from the externalsurface 228 and re-enter the substrate 64 through its lower surface 72.The inclination angle of surface 228 is set to compensate for the“missing” angle ε. Consequently, the light waves which are higher thanthe critical angle now having the incident angles of ε_(in)⁽⁰⁾+α_(Sur1), impinge again on the coupling-in surface 65 and arecoupled inside the substrate in a similar manner as illustrated above inrelation to FIGS. 14A-14C and 19A-19D.

In all the embodiments illustrated above, a high FOV of 40°, along thepropagation direction inside the substrate, was achieved utilizing asingle coupling-out surface 67. For side-view configurations, such aseyeglasses, the diagonal FOV can be 47° or 50°, depends on the aspectratio of the display source (9:16 or 3:4, respectively). For top-downconfigurations, such as helmet-mounted-displays, the diagonal FOV can beextended to more than 80° for aspect ratio of 9:16. Assuming, for thesake of maximizing the brightness efficiency, that a single coupling-outsurface in the substrate is preferred, there are two contradictingrequirements from the angular orientation α_(Sur1) of that surface. Onthe one hand, as a result of the limitation given in Eq. (7), it ispreferred to increase the angle in order to enlarge the total FOV thatcan be coupled inside the substrate. On the other hand, the extent ofthe output aperture 89 of the substrate is proportional tod·cot(α_(Sur1)), wherein d is the thickness of the substrate, namely,the output aperture, and therefore the EMB, will be extended by reducingα_(Sur1). It is also possible to increase the output aperture byincreasing the thickness of the substrate, but the input aperture willbe also increased accordingly. In addition, it is usually required thatthe substrate will be as thin as possible.

FIG. 21 illustrates a modified version of the embodiment shown in FIGS.14A-14D. Instead of using a single substrate 64, the shown system 150comprises two adjacent substrates 64 a and 64 b. The upper surface 70 bof substrate 64 b is optically attached to the lower surface 72 a ofsubstrate 64 a, defining an interface surface 152. The orientation angleα_(sur−b) of the coupling-in and the coupling-out surfaces 65 b and 67b, is set by the required FOV according to the limitation of Eq. (7),while the orientation angle α_(sur−a) of the coupling-in and thecoupling-out surfaces 65 b and 67 b, is set to a lower value of

$\begin{matrix}{\alpha_{{sur} - a} = {\alpha_{{sur} - b} - {\delta.}}} & (16)\end{matrix}$

As a result, the entire FOV can be coupled inside the lower substrate 64b. To withstand the requirement of Eq. (7), however, only a partial partof the FOV can be coupled inside the upper substrate 64 a. That is tosay, the FOVs coupled inside the two substrates are

$\begin{matrix}{{F^{(a)} = \left\{ {{\alpha_{\min}^{(0)} + {2 \cdot \delta}},\alpha_{\max}^{(0)}} \right\}};{F^{(b)} = {\left\{ {\alpha_{\min}^{(0)},\alpha_{\max}^{(0)}} \right\}.}}} & (17)\end{matrix}$

The lower part of the FOV {α_(min) ⁽⁰⁾, α_(min) ⁽⁰⁾2·δ, α_(max) ⁽⁰⁾} isthus coupled only inside the lower substrate 64 b, and to in order toavoid a cross-talk with the upper part of the FOV, it is not coupledinside the upper substrate 64 a. Since the light waves from the lowerpart of the FOV illuminate the viewer's eye from the left part of theoutput aperture, it should be coupled out from the left coupling-outsurface 67 b, that is, it should be transmitted to the eye only throughthe lower substrate 64 b. Therefore, the total FOV of {α_(min) ⁽⁰⁾,α_(max) ⁽⁰⁾} can be retained for the entire EMB. In addition, the outputaperture AP_(out) is expanded by the extent of

$\begin{matrix}{{\Delta\;{AP}_{out}} = {d_{a} \cdot {\left\lbrack {{\cot\left( \alpha_{{Sur} - a} \right)} - {\cot\left( \alpha_{{Sur} - b} \right)}} \right\rbrack.}}} & (18)\end{matrix}$

Alternatively, for a given output aperture, the thickness of the doublegrating d_(b)+d_(a) can be thinner by the ratio of

$\begin{matrix}{\frac{d_{b} + d_{a}}{d} = \frac{d \cdot {\cot\left( \alpha_{{Sur} - b} \right)}}{{d_{a} \cdot {\cot\left( \alpha_{{Sur} - b} \right)}} + {d_{a} \cdot {\cot\left( \alpha_{{Sur} - b} \right)}}}} & (19)\end{matrix}$

wherein d_(a) and d_(b) are the thicknesses of the substrates 64 a and64 b respectively, and d is the thickness of a single substrate such asin the embodiment illustrated in FIGS. 14A-14B. Consequently, theembodiment of FIG. 21 has the advantages of a wider FOV, determined bythe bigger angle α_(sur−b), as well as a larger output aperturedetermined by the smaller angle α_(sur−a). Since each one of the twosubstrates 64 a and 64 b functions independently, each separatesubstrate can have different parameters, in addition to the inclinationangle. The two substrates can have, inter alia, different thickness,refractive index and Abbe number, according to requirements of theoptical system. Moreover, the relative locations of the coupling-insurfaces 65 a and 65 b, as well as that of the coupling-out surfaces 67a and 67 b, can be set freely to minimize the input aperture 86′ (seeFIG. 25) and at the same time to maximize the output aperture 89 (seeFIG. 25) of the system.

As illustrated in FIGS. 22A, 22B and 22C respectively, three rays fromthe left marginal light wave 153 (153 a, 153 b, 153 c) are coupledinside the lower substrate 64 b after three reflections from surface 65b, one ray 153 d (FIG. 22B) is coupled after two reflections, and twoother rays, 153 e, 153 f (FIG. 22C) are coupled after a singlereflection from surface 65 b. As shown in FIG. 22D, all the rays arecoupled out from the substrate 64 b by the coupling-out element 67 b andare redirected to illuminate the entire EMB 100.

In FIGS. 23A, 23B and 23C respectively, there are illustrated two raysfrom the central light wave 154 (154 a, 154 b, 15 c) coupled inside thelower substrate 64 b after a single reflection from the surface 65 b andare coupled out by surface 67 b, two rays (154 c,154 d) are coupledinside the upper substrate 64 a after three reflections from surface 65a and are coupled out by surface 67 a, and two other rays 154 e, 154 f(FIG. 23C) are coupled inside the upper substrate 64 a after tworeflections from surface 65 a and are coupled out by the surface 67 a.As shown in FIG. 23D, all the rays are redirected by the redirectingprism 80 to illuminate the entire EMB 100.

FIG. 24A illustrates two rays from the right marginal light wave 155(155 a, 155 b) coupled inside the upper substrate 64 a after tworeflections from surface 65 a, and three other rays, 155 c, 155 d, 155 ecoupled after a single reflection from surface 65 a. As shown in FIG.24C, all the rays are coupled out from the substrate 64 a by thecoupling-out element 67 a and are redirected to illuminate the entireEMB 100. As illustrated in FIG. 25, the light waves in the entire FOVincident on surface 70 inside an input aperture 86′, which issubstantially smaller than the output aperture 89, illuminate the entireEMB.

Another issue that should be considered is ghost images that can be seenin an image as a result of undesired reflections of stray rays from theexternal surfaces of the system. As illustrated in FIG. 26A, an inputray 160 is coupled into the substrate 64 after a single reflection fromsurface 65 and is then coupled out from the substrate after a singlereflection from surface 67. The light ray is then partially reflected bysurfaces 79 i and 79 j as output rays 160 a and 160 b, into the viewer'seye at (in?) the “proper” direction. Part of the ray 160, however,passes-through surface 79 j, is totally reflected from the lower surface162 of prism 80, is then partially reflected from surface 79 k,passes-through substrate 64, is totally reflected from the upper surface70 of substrate 64, passes again through substrate 64, and then ispartially reflected from surface 79 m as an output ray 160 c into theviewer's eye at the “wrong” direction. That is to say, the stray ray 160c will appear as a ghost image in the projected image. FIG. 26Aillustrates such a ghost image which is originated from the coupled-inimage light waves. Other ghost images, however, can be initiated as aresult of light waves from the external scene. As illustrated in FIG.26B, an external ray 163 passes through a partially reflecting surface79 n, passes through prism 80 and the substrate 64 and reaches theviewer's eye at the original direction as ray 163 a. Part of ray 163,however, is partially reflected from surface 79 n, is totally reflectedfrom the lower surface 162 of prism 80, is partially reflected fromsurface 79 o, passes-through substrate 64, is totally reflected from theupper surface 70 of substrate 64, passes again through substrate 64, andthen is partially reflected from surface 79 p as an output ray 163 binto the viewer's eye at the “wrong” direction”. Hence, the stray ray163 b will also appear as a ghost image in the projected image.

As shown in FIGS. 26A and 26B, the main reason for the ghost images isthe undesired reflections from the surface 162. This phenomenon istypical not only for the embodiments illustrated in the presentapplication but also in other substrate-guided configurations. Unlikethese other configurations, the total internal reflection from surface162 is not required for the propagation of the light waves inside thesubstrate, and hence, it can be totally eliminated. A possible way toeliminate the undesired reflections from surface 162 is to apply anabsorptive layer to this surface. This simple method can be used fornon-see-through systems, wherein the external surface 162 can be totallyopaque. For see-through systems, however, since the light rays from theexternal scene should pass through surface 162 to reach the viewer's eye24, it is not permitted that surface 162 will be opaque.

FIG. 27 illustrates a more efficient method to remove the total internalreflection from surface 162, while keeping this surface substantiallytransparent to light rays from the external scene. As shown, the uppersurface 166 of a thin flat transparent plate 167 is optically attachedto the lower surface 162 of the redirecting prism 80. An array ofparallel absorptive surfaces 168 ₁, 168 ₂ . . . , oriented normal tosurface 166, is embedded inside the plate 167. To validate that all thelight rays that impinge on surface 162 will be absorbed by thesesurfaces, the following relation must be satisfied:

$\begin{matrix}{T \geq {0.5 \cdot D \cdot {\cot\left( \alpha_{\min}^{p} \right)}}} & (20)\end{matrix}$

wherein T is the thickness of plate 167, D is the distance between twoconsecutive surfaces 168 _(i) and 168 _(i+1), and α_(min) ^(p) is theminimal off-axis angle of the light waves impinging on plate 167. Asshown, ray 171 is absorbed by surface 168, after a total reflection fromthe lower surface 169 of plate 167, while ray 172 is absorbed by adirect impingement on surface 168 _(j). Since the substrate 64 is thinand the absorptive surfaces are normal to the major surfaces of thesubstrate, and hence, to the boresight of the viewer, plate 167 ispreserved substantially transparent to light rays from the externalscene.

FIGS. 28A to 28F illustrate a method for fabricating the plate 167. Aplurality of transparent flat plates 174, having a thickness of T arefabricated (FIG. 27). Since the major surfaces of these plates should beabsorptive, they should not necessarily be polished and theirparallelism is not crucial. A thin absorptive layer 175 is applied toone of the major surfaces of each plate (FIG. 28B). This absorptivelayer can be, inter alia, a black painting, a thin silicon coating, ametal coating or any other absorptive material that can be applied as athin layer. The plates 176 are cemented together using an appropriateoptical adhesive, so as to form a stack (FIG. 28C)). A number ofsegments 167′_(i) are then sliced off of the stacked form 176 (FIG. 28D)at a direction normal to the major surfaces of plates 174 _(i), and arethen processed by cutting, grinding and polishing, to create plates167″_(i), having a thickness of T′ (FIG. 28E). One of the major surfacesof the plate is optically cemented to surface 162 (FIG. 28F). In manycases it is required that plate 167 will be very thin, in an order of0.1 mm In that case, it might be difficult to process plate 167′, havingthe required thickness of T. Therefore, a plate having a thickness ofT′>T will be cemented to prism 80 and the lower surface 169′ of thecemented plate 167″ will be grounded and polished to achieve therequired thickness of T of the final plate 167.

FIGS. 29A and 29B illustrate embodiments similar to those shown in FIGS.26A-26B, wherein a plate 167 is optically attached to the lower surface162 of the prism 80. As shown, instead of being totally reflected fromsurface 162 and continue to propagate in the system, the stray lightrays 160 c and 163 b are absorbed in plate 167, and hence, the ghostimages, originated from the projected image as well as from the externalscene, are totally eliminated. This method for decaying ghost imagesresulting from undesired total internal reflection, could also beapplied to other optical modules, wherein stray light rays areundesirably reflected from a surface that should be otherwisetransparent to normal incident light. The plate 167 can thus beoptically attached to such a surface in order to decay the undesiredreflections while still maintaining the required transmittance of thesurface.

The advantages of reducing the lateral dimension of the input apertureas illustrated above, is even more apparent wherein two-dimensionalexpansion of the coupled light waves are required. FIG. 30 is aschematic drawing illustrating a way to expand the beam along two axesutilizing a double substrate configuration. For simplicity, theintermediate prisms and the redirecting elements were omitted from thedrawing. The input image 256 is coupled through the input aperture 274into the first substrate 264 a, which has a structure similar to one ofthe embodiments illustrated above, by the first reflecting surface 265a, and then propagates along the axis. The coupling-out element 267 acouples the light out of substrate 264 a through the output aperture 276and then the light is coupled into the second main substrate 264 b bythe coupling-in element 265 b through the input aperture, whichcoincides with the output aperture 276 of the first substrate 264 a. Thelight waves then propagate along the axis and are coupled out by thecoupling-out element 267 b through the output aperture 278. As shown,the original image 256 is expanded along both axes, where the overallexpansion is determined by the ratio between the lateral dimensions ofthe apertures 274 and 278. As shown, each light wave (represented by asingle arrow in the drawing) illuminates only part of the outputaperture 278, but all the light waves are coupled out having therequired directions into the EMB 100.

In all of the above embodiments, it has been assumed that the displaysource is unpolarized. There are micro-display light sources, however,such as LCDs or LCOS, wherein the light is linearly polarized, and thiscan be used to make a more compact collimating system. As illustrated inFIG. 31A, the p-polarized input light waves 107L, 107M and 107R from thedisplay light source 4, are coupled into a light guide 279, usuallycomposed of a light-waves transmitting material, through its surface280. The light waves pass through the polarizing beamsplitter 282 andare coupled out of the light guide 279 through surface 283. The lightwaves then pass through a quarter-wavelength retardation plate 285,collimated by a lens 286 at its reflecting surface 289, return to passagain through the retardation plate 285, and re-enter the light guide279 through surface 283. The now s-polarized light waves reflect off thepolarizing beamsplitter 282 and exit the light guide through the lowersurface 290. The light waves are now coupled into the substrate 64through the intermediate prisms 226 and 220, in the same manner asillustrated above in relation to FIGS. 20A-20D. The reflecting surface289 can be materialized either by a metallic or a dielectric coating.

Utilizing a reflecting collimating lens 286, as illustrated in FIG. 31A,has some prominent advantages, such as achieving good performance byusing a small number of optical components, having additional compactcollimating modules, etc. It is therefore advantageous to use thisembodiment also for unpolarized light sources such as Micro-LEDs andOLEDs. The main drawback in such a case is that only a singlepolarization component of the display source can be used, and hence,achievable brightness is reduced by more than 50%. An alternative methodfor utilizing the two orthogonal polarization components of anunpolarized display source, and therefore avoiding the brightnessreduction, is illustrated in FIG. 31B. As shown, the s-polarizedcomponents of the input light-waves 107L, 107M and 107R from the displaylight source 4 are coupled into a light-guide 279, through its rightsurface 280. Following reflection-off of a polarizing beamsplitter 282,the light waves are coupled-out of the substrate through surface 291 ofthe light-guide 279. The light-waves then pass through a secondquarter-wavelength retardation plate 293, collimated by a second lens296 at its reflecting surface 297, return to pass again through theretardation plate 293, and re-enter the light-guide 279 through surface291. The now p-polarized light-waves pass-through the polarizingbeamsplitter 282, exit the light-guide through the lower surface 290,and are coupled into the substrate 64 through the intermediate prisms226 and 220 as before. The p-polarized component of the light source iscoupled into the substrate as illustrated in FIG. 31A. The twocollimating lenses should be identical and be placed very accurately atthe surfaces of the light-guide 279 in order to avoid double image.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

In particular it should be noted that features that are described withreference to one or more embodiments are described by way of examplerather than by way of limitation to those embodiments. Thus, unlessstated otherwise or unless particular combinations are clearlyinadmissible, optional features that are described with reference toonly some embodiments are assumed to be likewise applicable to all otherembodiments also.

1. An optical device, comprising: a first light-transmitting substratehaving at least two parallel major surfaces and two opposite edges; aninput aperture; an output aperture positioned next to one of the majorsurfaces of the substrate; an eye-motion box having an aperture; a firstintermediate element having at least two surfaces positioned outside ofthe substrate for coupling incoming light waves, having a field-of-view,into the substrate through the input aperture; a first flat reflectingsurface, having an active area located between the two major surfaces ofthe light-transmitting substrate, for reflecting the incoming lightwaves from the first intermediate element to effect total internalreflection from the major surfaces of the substrate; a second flatreflecting surface parallel to the first flat reflecting surface, havingan active area and being located between the two major surfaces of thelight-transmitting substrate, for coupling light waves out of thesubstrate, and a redirecting optical element having at least twosurfaces positioned outside of the substrate for redirecting light wavescoupled-out from the substrate through the output aperture, into theeye-motion-box, wherein the input aperture is substantially smaller thanthe output aperture, all the incoming light waves pass inside the inputaperture, the active area of the first reflecting surface is similar tothe active area of the second reflecting surface, and each of thecoupled light waves covers the entire aperture of the eye-motion-box. 2.The optical device according to claim 1, wherein the light waves coupledinto the substrate and out of the substrate have a brightness, thebrightness of the light waves coupled-out from the substrate by thesecond flat reflecting surface, is substantially similar to thebrightness of the light waves coupled into the substrate.
 3. The opticaldevice according to claim 1, wherein the first and the second flatreflecting surfaces couple the light waves into and from the substrate,respectively, by total internal reflection.
 4. The optical deviceaccording to claim 1, wherein light waves coupled inside the substrateare reflected the same number of reflections from the first and thesecond flat reflecting surfaces.
 5. The optical device according toclaim 1, wherein light waves passing through the input aperture andcoupled into the substrate are incident only on part of the first andthe second flat reflecting surfaces.
 6. The optical device according toclaim 5, wherein light waves passing through the input aperture and theaperture of the eye-motion-box are incident only on the part of thefirst reflecting surface closer to one of the edges of the substrate andare coupled out from the substrate only by a part of the secondreflecting surface closer to the other edge of the substrate.
 7. Theoptical device according to claim 5, wherein light waves passing throughthe input aperture and the aperture of the eye-motion-box are incidentonly on the part of the first reflecting surface closer to the center ofthe substrate and are coupled out from the substrate only by a part ofthe second reflecting surface which is closer to the center of thesubstrate.
 8. The optical device according to claim 5, wherein lightwaves passing through the input aperture and the aperture of theeye-motion-box are incident only on a central part of the firstreflecting surface and are coupled out from the substrate only by acentral part of the second reflecting surface.
 9. The optical deviceaccording to claim 1, further comprising a second intermediate element,wherein the light waves pass through the first and the secondintermediate elements, prior to being coupled into the substrate by thefirst reflecting surface.
 10. The optical device according to claim 1,wherein the coupled-in light waves pass through the first flatreflective surface at least twice prior to being reflected by thesurface to be coupled into the substrate.
 11. The optical deviceaccording to claim 1, wherein the first intermediate element and theredirecting optical element are optically cemented by a first opticaladhesive to the major surfaces of the substrate having a refractiveindex, define first and second interface planes, wherein the refractiveindex of the adhesive is substantially lower than the refractive indexof the substrate, and an anti-reflection coating is applied to theinterface planes.
 12. The optical device according to claim 11, whereinthe interface planes are substantially transparent for light waveshaving incident angles lower by more than one degree than the criticalangle of the interface planes.
 13. The optical device according to claim1, wherein a second optical adhesive is applied to the first and thesecond reflecting surfaces, the refractive index of the adhesive issubstantially lower than that of the substrate, anti-reflection coatingsare applied to the reflecting surfaces, and the surfaces aresubstantially transparent for light waves having incident angles lowerby more than one degree than the critical angle of the surfaces.
 14. Theoptical device according to claim 12, wherein the first intermediateelement and the redirecting optical element are fabricated from the sameoptical material having a refractive index and Abbe number,substantially different than those of the substrate, creating a firstchromatic dispersion of the light waves coupled inside the substrate.15. The optical device according to claim 14, wherein the Abbe number ofthe first adhesive is substantially different than that of thesubstrate, creating a second chromatic dispersion of the light wavesinside the substrate, and the first and the second chromatic dispersionsare substantially mutually compensated.
 16. The optical device accordingto claim 1, further comprising a second light transmitting substratehaving at least two major surfaces, two opposite edges, and a third anda fourth flat reflecting surfaces parallel to each other, each of thesurfaces having an inclination angle, wherein the two substrates areoptically attached and the inclination angle of the third and the fourthflat reflecting surfaces to the major surfaces of the second substrate,is lower than the inclination angle of the first and the second flatreflecting surfaces, to the major surfaces of the first substrate. 17.The optical device according to claim 1, further comprising a secondlight transmitting substrate having at least two major surfaces, twoopposite edges and third and fourth flat reflecting surfaces, the inputand output apertures having lateral dimensions, wherein light wavescoupled out from the first substrate are coupled into the secondsubstrate and the lateral dimensions of the input aperture aresubstantially smaller than the lateral dimensions of the output aperturealong two different axes.
 18. The optical device according to claim 1,further comprising a flat plate optically attached to a surface of theredirecting optical element, wherein an array of flat absorptivesurfaces, substantially normal to the surface of the redirecting opticalelement, are embedded inside the plate, the flat plate is substantiallytransparent to normal incident light waves, and light waves coupled outfrom the substrate and incident on the flat plate are absorbed by theabsorptive surfaces.
 19. The optical device according to claim 1,wherein the first intermediate element and the redirecting element areoptically cemented by an optical adhesive to the major surfaces of thesubstrate having a refractive index, defining a first and a secondinterface planes, anti-reflection coatings and a thin film dielectriccoating are applied to the interface planes wherein the refractive indexof the dielectric coating is substantially lower than the refractiveindex of the substrate.
 20. The optical device according to claim 1,wherein the substrate has a refractive index and the surfaces have acritical angle, the first and second reflecting surfaces have a thinfilm dielectric coating and a refractive index, the refractive index ofthe dielectric coating is lower than the refractive index of thesubstrate, the reflecting surfaces having anti-reflection coatings whichare transparent to the light waves having incident angles lower than thecritical angle of the surfaces.